The present invention relates to a carriage moving device for moving a carriage having a head unit for writing and/or reading a signal to a disc-like information recording medium in a radial direction of the disc-like recording medium.
In a disc driving device for carrying out recording and/or reproducing of information to an information recording medium, it is necessary to move a magnetic head for writing and/or reading a signal in a radial direction of the recording medium. A moving device for moving the magnetic head requires to be equipped with a motor as a driving source and a transmitting mechanism for transmitting a driving power of the motor to a carriage supporting the magnetic head. FIG. 40 shows an example of a conventional carriage moving device of a screw shaft type. Referring to FIG. 40, reference numerals 150 and 151 designate a housing and a stepping motor, respectively. A casing 152 of the stepping motor 151 is fixed to the housing 150 by screws 153. There are incorporated in the casing 152 a motor stator constituted of a coil 155 wound around a coil bobbin 154 and a comb-type yoke 156 integrally formed with the casing 152, and a motor rotor constituted of a cylindrical permanent magnet 157.
Reference numeral 158 designates a screw shaft formed with a spiral groove 158a on an outer circumferential surface thereof. The screw shaft 158 is inserted into the permanent magnet 157, and is fixed thereto. The screw shaft 158 is rotatably supported at its both small-diameter conical ends by bearings 159 and 160 having ball bearings. The bearing 159 is retained by a retainer 162 to the housing 150, and is normally biased to the screw shaft 158 side by a spring 163 incorporated in the retainer 162. Accordingly, the screw shaft 158 is normally biased to the bearing 160 side. The bearing 160 is mounted to an adjusting screw 165 screwed into a support member 164 fixed to the stepping motor 151. The screw shaft 158 is designed to be finely adjustable in the thrust direction by rotating the adjusting screw 165.
A carriage 168 having a writing and/or reading magnetic head (not shown) is adapted to be moved along guide rods 169. The carriage 168 includes an engagement member 170 having a conical edge portion 170a resiliently engaged with a spiral groove 158a of the screw shaft 158. Accordingly, when the screw shaft 158 is rotated in forward and reverse directions by the stepping motor 151, the edge portion 170a of the engagement member 170 engaged with the spiral groove 158a is moved to reciprocate the carriage 168 along the same direction as the axial direction of the screw shaft 158 by the amount corresponding to the rotative quantity of the screw shaft 158.
In the moving mechanism as mentioned above, the housing 150 is formed with a lateral hole 150a, a lateral bore 150b and screw holes 150c so as to mount the stepping motor 151, and the two guide rods 169 are arranged in parallel relation with the screw shaft 158 of the stepping motor 151. The guide rods 169 are mounted at their both ends to grooves (not shown) formed at the housing 150. The grooves are accurately formed to function as a reference plane for maintaining the height of the carriage 168 constant. Therefore, the carriage 168 may be moved along a plane formed by the two guide rods 169 at all times irrespective of the engaged position of the edge portion 170a of the engagement member 170 with the spiral groove 158a of the screw shaft 158. A similar construction is disclosed in Japanese Laid-Open Utility Model Publication No. 60-64470 filed by the same applicant as in this application.
Recently, it has been intensively required to make the disc driving device thin. However, in the conventional carriage moving mechanism, the thickness of the disc driving device is dependent upon an outer diameter of the stepping motor 151. Accordingly, the outer diameter of the stepping motor 151 is required to be reduced, so as to make the disc driving device thin. However, if the outer diameter of the stepping motor 151 is reduced, a driving torque of the motor is reduced. To obtain a required driving torque, it is necessary to elongate the rotor and the stator along the rotating shaft. As a result, the length of the stepping motor is increased to result in reduction in a space factor. In this manner, there is a limit of reduction in thickness and size of the conventional disc driving device.